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Is it possible to get higher load of energy into a battery if you charge it when its cold eg. +9C? the battery is emptier, but does it let the power in? Is it safe?

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In general a battery can deliver a certain number of electrons before discharging. This is because the electrons are generated by a chemical reaction and there are a fixed number of molecules/atoms/whatever reacting.

The power depends on the voltage drop the electrons flow through as the battery discharges. Generally speaking the voltage of batteries decreases as the temperature decreases, so the power a battery can deliver is reduced at low temperature and increased at high temperature.

Charging is just discharging in reverse, so at low temperatures it will take less electricity from your mains to fully charge a battery than it will at high temperature. However the charge held by the battery will end up the same regardless of temperature.

All the above are general comments, and various different kinds of batteries will have restrictions based on how they're made. I'd guess you're talking about car batteries, i.e. lead-acid, batteries, and these are pretty tolerant of temperature within reason. I believe you need to use a lower charge rate below $0^\circ$C though I confess I'm not sure exactly why. To answer your specific question: charging your car battery at $9^\circ$C is fine and will be quicker than at say $20^\circ$C but the final charge of the battery won't be any different.

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You say that charging the battery at low temperature takes less electricity. So if I charge it at low temperature, then heat it, and use it to power something, it'll release more energy than I charged in? Where the difference in energy comes from? Just from heating the battery? – Petr Pudlák Aug 17 '12 at 6:59

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